Generally, a “case hardening treatment” such as induction quenching, carburizing-quenching, carbonitriding-quenching, and the like is applied to steel components of automobiles and industrial machinery, in particular, for gears, pulleys, shafts and the like made of steel which are used as power transmitting components.
Among those described above, the “induction quenching” is a quenching in which steel components are rapid heated into a high-temperature austenite region with a temperature of the Ac3 points or higher, and thereafter cooled. The induction quenching has an advantage in that the adjustment of case depth is relatively easy. In order to obtain a necessary surface hardness, case depth, and core hardness, it is general to use, as the material to be treated, a medium carbon steel such as S45C specified in JIS G 4051 (2009) and SCr440 specified in JIS G 4053 (2008).
However, since a medium carbon steel has a higher material hardness than that of a low carbon steel, it is worse in machinability in cutting process as well as in cold forgeability during net-shape forming. In addition, in the case of induction quenching, a problem exists in that an induction heating coil needs to be made for each component.
For that reason, the carburizing-quenching or carbonitriding quenching has become more frequently used for the case hardening treatment of the steel components mentioned above.
When the carburizing-quenching or carbonitriding quenching is used as the case hardening treatment, the steel components mentioned above are produced, for example, by the following method.
(1) A rolled steel bar or wire rod made of steel for machine structure is prepared. As the steel for machine structure, a steel which has a lower C content than that of a steel used for induction quenching, such as SCr420, SCM420, and SNCM420 etc. specified in JIS G 4053 (2008), is used.
(2) The prepared rolled steel bar or wire rod is hot forged to be roughly formed into an intermediate product.
(3) The roughly formed intermediate product of (2) described above is subjected to cutting after being subjected to normalizing treatment as needed.
(3′) The roughly formed intermediate produce of (2) described above is subjected to net-shape forming by cold forging after being subjected to normalizing treatment as needed.
(4) The intermediate product which has been subjected to cutting or net-shape forming is subjected to carburizing-quenching or carbonitriding-quenching as the case hardening treatment and further to tempering at a temperature not more than 200° C. as needed, to obtain the steel components described above.
(5) Shotpeening and/or surface grinding may be further performed after the case hardening or after the tempering of (4) described above to obtain the steel components.
In recent years, to improve the fuel economy of automobiles and industrial machines, or to realize higher output power of engines, reduction in weight and size of steel components has been promoted. However, as a result of such reduction in weight and size, load applied to the steel components tends to increase. For that reason, improvements in the bending fatigue strengths in a high-cycle region, as well as improvements in bending fatigue strength in a low-cycle region and pitting resistant strength is demanded for the steel components.
To be specific, for example, in the case of a gear for automobiles, at a gear tooth root, higher bending fatigue strength in a high-cycle region of a number of load repetitions of about 1.0×107 is demanded in respect of suppressing a tooth root breakage, and also higher bending fatigue strength in a low-cycle region of a number of load repetitions of about 1.0×105 is demanded in respect of suppressing a tooth root breakage at a large load applied at the start of driving. Further, at a tooth face, higher pitting resistant strength is demanded in respect of suppressing noises during meshing of gears and suppressing a breakage of tooth starting at a portion of exfoliation.
Hereafter, the bending fatigue strength in a high-cycle region as described above is referred to as “high-cycle bending fatigue strength”, and the bending fatigue strength in a low-cycle region is referred to as “low-cycle bending fatigue strength”.
To meet such demands, a technique of performing a case hardening treatment by carburizing-quenching or carbonitriding-quenching using a steel containing a large amount of alloying elements compared with the steel for machine structure specified in JIS G 4053 (2008) described above, and a technique of further performing shotpeening after case hardening treatment have been proposed.
Patent Document 1 (JP6-306572A) discloses a “gear” made of a material which contains elements such as Si: not more than 0.1%, Ni: 0.4 to 0.6%, Mo: 0.6 to 1.0%, and Nb: 0.02 to 0.5% with the balance being Fe, and has a carburized abnormal layer of not more than 6 μm and a grain size of No. 9 or larger.
Patent Document 2 (JP64-31927A) discloses a “method for producing a heat treated steel component” in which a steel consisting of C: 0.10 to 0.40%, Si: not less than 0.06% and less than 0.15%, Mn: 0.30% to 1.00%, Cr: 0.90% to 1.20%, Mo: more than 0.30% and not more than 0.50%, the balance being Fe, is subjected to carburizing-quenching or carbonitriding-quenching, and subsequently to shotpeening.
Patent Document 3 (JP60-21359A) discloses a “steel for gear to be used with carburizing treatment” which contains elements such as Cr: 0.40 to 1.50% and Si: not more than 0.10% and further contains, as needed, one or more kinds of Ni: not more than 2.50%, Mo: not more than 0.40%, and Nb: 0.005 to 0.025%, with the balance being substantially Fe.
Patent Document 4 (JP7-242994A) discloses a “steel for gear having excellent tooth face strength” which contains elements such as Si: not more than 1.0% and Cr: 1.50 to 5.0% with the balance being Fe and impurities, and a “method for producing a gear having excellent tooth face strength” in which a case hardening treatment by carburizing-quenching and tempering or carbonitriding-quenching and tempering is performed using the aforementioned steel for gear, or shotpeening is further performed, as needed, after the case hardening treatment.
Patent Document 5 (JP7-126803A) discloses a “steel for carburized gear” which contains elements such as Si: 0.35 to 3.0%, Cr: 0.3 to 5.0%, and V: 0.05 to 0.5% and further contains, as needed, one or more kinds of Ni: not more than 3.0%, Mo: not more than 1.0%, and Nb: not more than 0.1%, with the balance being Fe and inevitable impurities.